Equilibrium constants of the formation of gas-phase hydrogen bonded molecular complexes have been determined using a method combining experimental and theoretically simulated vibrational spectra. The equilibrium constants are determined from the monomer pressures and the pressure of the complex. The pressures of the monomers are measured experimentally, but the pressure of the complex is too low to detect. However, a combination of an experimentally measured integrated absorbance from Fourier-Transform Infrared spectra and a theoretically calculated oscillator strength allows us to estimate the pressure of the complex and determine the equilibrium constant of the complex formation. We have developed a vibrational model to improve the accuracy of the calculated oscillator strengths, which is used to determine the pressures of the molecular complexes, thus improving the accuracy of the equilibrium constants. Our model and results obtained using this model will be presented and discussed